Receiver control circuit



' SOURCE p 5, 1939- I R. B. ARMSTRONG 2,171,639

RECEIVER CONTROL 0 IRCUIT Filed March 5, 1938 7 2 ram; NETWORK g i 17 ram c RECTIFIER K T0 S/GIVAL 70 A. E NETWORK A TTORNEY.

Patented Sept. 5, 1939 UNITED STATES P A'i l i'i @lTiFi iZIJI RECEIVER, CONTROL CIRCUIT Application March 3,

1938, Serial No. 193,616

In Great Britain April 6, 1937 3 Claims.

This invention relates to radio, and other carrier, wave receivers, and more particularly to receivers of the kind employing automatic gain control circuits and detector circuits with controllable reaction between output and input.

The use of controllable reaction between the anode and grid circuits of a detector provides very important advantages in many types of receiver, among the benefits being that of providing simple control of, and improvement in, gain and selectivity. Further, if the reaction be sufficient to cause the detector to be a self-oscillating detector, it provides what is probably the simplest known form of beat reception of unmodulated signals. Hitherto, however, the use of detectors with controllable reaction has presented two serious disadvantages-especially where-the detector is the second detector in a superheterodyne receiverif voltage for automatic anti-fading, or like gain control, is obtained from the detector input circuit.

In the drawing:

Fig. 1 shows a known type of circuit to illustrate the problems solved by this invention,

Fig. 2 shows a circuit employing the invention.

The aforesaid two disadvantages will be best appreciated by considering the simple known circuit shown diagrammatically in Fig. 1. Here the anode l of a carrier frequency valve 2 (which may be a high frequency, or an intermediate frequency valve) is tapped at 3 upon a parallel tuned circuit 4, 5, between the grid 6 and cathode l of a detector tube 8; for which detector controllable reaction is provided by means of a circuit consisting of an adjustable reaction condenser ii in series with an inductance IQ. The

latter is coupled to the inductance t in the parallel tuned circuit 0, 5; the two series elements 9, iii, being connected between the anode II of the detector valve and the low potential end l2 of the tuned grid circuit 4, 5. Suppose that in such a circuit employing controllable reaction, an automatic gain control system (not shown) as known per se is provided for controlling the gain of the valve, or valves, preceding the detector, and that this system is operated by means of an input coil l3 (hereinafter termed the A. V. C. coil) coupled as indicated by the arrow to the coil 4.

To take a practical example, suppose that the whole receiver is such that a voltage of the order of of :1 volt is produced by a wanted signal on the grid 6. Now with such an arrangement the very small voltage available in the A. V. C. coil is requires considerable amplification before it can furnish eflicient automatic gain control, and the amplifier circuits necessary for this are wasted insofar as they add nothing to the selectivity of the receiver as a whole. Further, if the reaction be increased by varying the reaction condenser 9 this will cause the automatic control system to operate, thus reducing the gain of the preceding portion of the receiver by an amount roughly equal to the increase in gain caused by the increase in reaction alone. The effect (as respects gain) of altering the reaction condenser 9 is practically zero until reaction is increased so far that the detector self-oscillates, whereupon the receiver becomes dead to all but very strong signals.

Another defect met with in known detector arrangements with controllable reaction is that, when made to self-oscillate, they can provide a beat note, or heterodyne, output only for comparatively weak signals. Measurement has shown that in a typical known circuit such as that shown in Fig. 1, beat reception of a signal which provides on the grid 5 half the voltage of the local oscillation is only possible if the circuit consisting of the reaction condenser 9 and the inductance if] in series therewith is mis-tuned by at least 0.3 percent. Thus, if the local oscillatory voltage on the grid 6 is 1 volt and an incoming signal provides half a volt at the same time, and the incoming frequency is 1000 k. c., then the lowest beat frequency output obtainable is of the order of 3 k. c., and that may in many cases be too high. For higher incoming frequencies or greater input strength the beat note obtainable must be still higher in frequency. This effect is well known, and is sometimes termed wiping or frequency lock effect. It often happens in practice that if such a detector is set in its most sensitive condition for the best reception of weak unmodulated signals, 1. e. it is just self-oscillating, a strong incoming signal may be missed altogether owing to there being produced no clear beat note.

The principal object of the present invention is to avoid these defects, and this object is achieved by loosely coupling the input circuit of the automatic gain control system to the input circuit of the detector having controllable reaction, and feeding the input circuit of the automatic gain control arrangement from an additional interposed valve stage of such amplification that despite said loose coupling, the voltage produced in the input circuit of the detector is substantially the same as it would be were said additional stage omitted and the detector input stage fed (as in the hitherto usual way) by tight coupling with the output circuit of a valve stage immediately preceding said additional stage.

Fig. 2 shows diagrammatically one way of carrying out this invention; in Figs. 1 and 2 like parts are indicated by like references. As Will be seen in Fig. 2 there is provided an additional high, or. intermediate, frequency (as the case may be) tuned valve stage including valve l4. Thus additional stage has its input circuit l5 fed from the output circuit of valve 2, i. e. of the valve which, in the known arrangement of Fig. 1 is the last high,'or intermediate, frequency stage. The output circuit of the valve I4 is tightly coupled by a coil IE to a tuned circuit l3, [1; the latter feeds into, and operates, the automatic gain control circuit system (not shown). The input circuit of the detector 8 (the detector circuit per se is the'same as the known circuit of Fig. 1) is loosely coupled to the A. V. C. coil. The arrangement is such that a wanted signal will produce on the grid [8 of the addtional valve I4 a voltage of the magnitude normally required for the detector input grid 6 e. g., a voltage of the order of onetenth of a volt. This voltage is amplified by the additional valve I4 so that there is available across the A. V. C. coil I3 a voltage of the order of, say, 3 volts. This is normally adequate to give satisfactory gain control (when rectified) without further amplification. The grid'circuit of the detector valve is, as stated, loosely coupled to the A. V. C. coil I3, the coupling being such that the detector valve grid 6 also receives a voltage of the cause variations in selectivity and sensitivity of the receiver as a whole; practically as though there were no automatic gain control system. Again, the voltage developed by an incoming signal on the grid of the detector valve is substantially independent of incoming signal strength. In the case of a self-oscillating detector it can be restricted to a valve which will give good beat reception. It is worthy of note that the additional stage provided in carrying out this invention is not wasted as respects selectivity, for it adds to the selectivity of the receiver as a whole.

What is claimed is:

l. A radio or like receiver including a demodulating detector, an adjustable feed back circuit between the output and input electrodes of said detector, an automatic gain control system operating automatically to control the carrier or high frequency gain of the receiver, at least two carrier or high frequency valve stages in cascade and preceding said detector, a loose coupling arrangement for transferring energy'from the output circuit of the second of said two stages to the input circuit of the detector, and a substantially tighter coupling between the said output circuit and the input circuit of the automatic gain control system.

2. A receiver as defined in claim 1 wherein the adjustable feed back circuit includes an adjustable condenser and a fixed inductance in series between the output electrode of the detector and the input electrode thereof.

3. A receiver as defined in claim 1 wherein the loose coupling is such that the voltage set up at the detector inputelectrode is substantially the same as that at the input electrode of the second of said two cascade stages and only a small fraction of that set up in the input circuit of the automatic gain control system.

REGINALD BASIL ARMSTRONG. 

